1. Field of The Invention
This invention relates to liquid crystal compositions and more particularly, to ferroelectric liquid crystal compositions which are adapted for use in ferroelectric liquid crystal display devices.
2. Description of The Related Art
In recent years, attention has been paid to ferroelectric liquid crystals (FLC), which make use of the switching effect of chiral smectic C phase (SmC* phase), as a liquid crystal material for use in a variety of liquid crystal display devices.
Such FLC display devices are advantageous in (1) high speed response (greater by 1000 times than that of conventional nematic liquid crystal display devices), (2) wide viewing angle, and (3) image memorizing properties.
A number of ferroelectric liquid crystal molecules have been prepared up to now. Since the response speed, .tau., of FLC is expressed according to the following equation, EQU .tau.=.eta./Ps.multidot.E
wherein .eta. represents a viscosity of material, Ps represents spontaneous polarization, and E represents an electric field, the liquid crystal molecules have been designed so that the spontaneous polarization is made large and the viscosity is made smaller in order to increase the response speed. By this, the value of .tau. has been considered to become small.
Although most systems are in conformity with the above equation with respect to the response speed, there arise several problems that when Ps is made larger, an electric field which is induced by liquid molecule's polarization itself directing opposite to an external electric field is generated in the liquid crystal display panel and that the alignment of molecules is disturbed owing to adsorption of impurity ions. This does not result in high speed response as would be expected, with a contrast being not satisfactory. Accordingly, it is a recent trend to set up some questions on the spontaneous polarization being made excessively large.
On the other hand, the following studies have been made on the realization of low viscosity. In order to extend the temperature range, it is usual to use, without use of chiral molecules alone, compositions which comprise mixtures of chiral molecules having the spontaneous polarization as a dopant and non-chiral molecules, as a base material, which are free of spontaneous polarization. In the composition, the base material is made higher in concentration in order to control the viscosity and the extension of the temperature range, so that the viscosity control ordinarily relies on the molecules of the base material. This is a so-called dopant method.
However, the dynamics of the electric field response of FLC are controlled by means of the chiral molecules having the spontaneous polarization. It is of no doubt that what type of chiral molecule is used is an important factor of determining the characteristics of the FLC display device. Accordingly, the emphasis on the control of the viscosity may lead to an adverse influence on the important characteristics of FLC with respect to the composition comprising a major proportion of base molecules.
With respect to the response speed, choice of material has been hitherto based on the above-indicated equation. However, not only some doubts now arise on the manner of the choice, but also any guideline on the contrast of the display has never been proposed.